The present invention relates to a method for manufacturing a turbine blade by joining a plurality of members, and particularly to a method for manufacturing a turbine blade formed of a nickel base heat-resistant alloy, and a turbine blade manufactured thereby.
In general, the gas turbine blade is in many cases manufactured by using a nickel base heat-resistant alloy and precision casting in order to cope with higher temperatures. In this case, many techniques of providing a complicated cooling passage in the inside of the turbine blade for cooling with a cooling gas (mainly, air) are applied.
The cooling passage is formed by use of a core at the time of casting, and the core support hole is left in the cast blade, so that the airfoil tip is in an opened condition. Therefore, in a gas turbine using a blade cooling system of the closed type in which the cooling gas is not allowed to flow out of the blade, the core support hole left at the airfoil tip must be sealed off to obtain a product. As a method of sealing the core support hole, arc welding, laser beam welding, electron beam welding and the like are used. However, the nickel base heat-resistant alloy forming the turbine blade is susceptible to crack due to lowering of ductility of grain boundary at the time of welding, and, therefore, it is difficult to obtain a sound weld.
In addition, since the turbine blade is worn with the use thereof and allowed to thin down, the worn turbine blade is replaced with a new blade at the time of overhaul or the like. The worn blade is subjected to repair for recovering the consumed portion to its original size, whereby a reusable blade is manufactured. However, also in the case of overlaying which is used for the repair, there is the problem of the weld crack mentioned above and it is difficult to obtain a sound weld.
As a method for welding the turbine blade without generating the weld crack, methods of lowering the crack susceptibility by controlling the composition of the weld metal are disclosed in Japanese Patent Laid-open Nos. 1-107973 (1989), 2001-90502, and 2001-158929 and the like. Besides, methods of controlling the temperature of the material to be welded so as to alleviate thermal stress and to thereby prevent cracking are disclosed in, Japanese Patent Laid-open Nos. 5-192785 (1993), 6-198438 (1994), and 2001-269784 and the like.
On the other hand, as a method for joining the turbine blade members other than welding, methods in which an insert metal having a fusion temperature lower than that of the blade members to be joined is interposed between the blade members, and the insert metal is melted by heating it to a temperature lower than the fusion temperature of the blade members and higher than the fusion temperature of the insert metal to cause diffusion bonding through utilization of a liquid phase generated between the blade members are disclosed in Japanese Patent Laid-open Nos. 5-31588 (1993) and 2000-263247. By these methods, joining without causing cracking can be achieved.
According to the welding methods described in the above-mentioned prior arts, though the cracking generated at the time of welding of the turbine blade members can be prevented, strain aging cracking may occur at a portion of the weld in the case of conducting a heat treatment such as an aging heat treatment after welding, possibly leading to a lowering in production yield.
In addition, since the portion having undergone fusion and solidification is different from the original blade member in crystal structure, there is the problem that properties such as strength are lowered. This effect is particularly conspicuous in the case of crystal orientation controlled alloys such as a single crystal alloy and a directionally solidified alloy.
According to the diffusion bonding methods utilizing the liquid phase described in the prior arts, also, though the cracking due to the bonding can be prevented, a lowering in strength due to the components of the insert metal at the joint may occur. To obviate this problem, it is necessary to perform a sufficient diffusion treatment at high temperatures, thereby making uniform the distributions of component elements contained in the joint. Therefore, there are the problems that the microstructure of the blade members is changed due to long-time heating and the original properties thereof may be damaged, or that the period of time required for the joining step is long and productivity is lowered.
It is an object of the present invention to provide a method for manufacturing a turbine blade by which cracking due to joining would not easily be generated and joining can be achieved at lower temperatures and in a shorter time as compared with the liquid phase diffusion bonding, and a turbine blade manufactured by the method.
The present invention resides in a method for manufacturing a turbine blade by manufacturing an airfoil portion of a turbine blade comprising the airfoil portion and a dovetail portion through joining a plurality of members, comprising the steps of: bringing the members to be joined into contact with each other, and applying pressure to the members to keep the contact state thereof; and applying a pulse voltage to the members to pass an electric current therethrough so as to heat the bonding interface by resistance heating for diffusion bonding.
Modes of the present invention will be described below.
In a first method, a pulse voltage is applied to a plurality of members to pass an electric current therethrough while applying a predetermined pressure to the bonding interface between the members, and the joint portion is heated in a predetermined temperature range for a predetermined time by resistance heating generated at the bonding interface and in the bulk of the members, whereby the members are joined to each other.
A second method is a method of forming a turbine airfoil portion by joining a primary member of turbine airfoil portion having a cooling passage therein and an airfoil tip member having the function of sealing the cooling passage. In this method, a pulse voltage is applied to the members to be joined to pass an electric current therethrough while applying a predetermined pressure to the bonding interface, and the joint portion is heated in a predetermined temperature range for a predetermined time by resistance heating generated at the bonding interface and the bulk of the members, whereby the members are joined to each other.
In a third method, the joint portion is heated in a predetermined temperature range for a predetermined time by heating by an auxiliary heating means other than the passing of electric current, whereby the members are joined to each other.
In a fourth method, the members to be joined are vibrated by an ultrasonic vibrator during the heating for joining the members.
The pressure applied to the bonding interface at the time of joining is preferably in the range of 10 to 99 t based on the tensile strength at the heating temperature of the member that has the lowest tensile strength at the heating-maintaining temperature. The heating temperature for the joint portion is preferably in the range of 0.6 to 0.99 times the fusion temperature of the member that has the lowest fusion temperature. The heating of the joint portion is preferably maintained for a period of time in the range of 60 to 7200 sec. The joining is preferably performed in a vacuum atmosphere of 10xe2x88x923 to 10xe2x88x927 Torr or in an inert gas atmosphere.
The method according to the present invention is most effective where the members constituting the turbine blade are each formed of a nickel base superalloy. It is extremely preferable that the members constituting the turbine blade are formed of nickel base superalloys, all or part of the members are formed of crystal orientation controlled alloys having the same crystal structure, and a difference in  less than 100 greater than  crystal orientation between the crystal orientation controlled alloys at the joint portion of the crystal orientation controlled alloy members is within 5xc2x0.
In order to ensure that all or part of the members constituting the turbine blade are formed of crystal orientation controlled nickel base alloys having the same structure and the difference in  less than 100 greater than  crystal orientation between the nickel base alloys at the joint portion is within 5xc2x0, it is preferable to crop the crystal orientation controlled alloy members constituting the blade from a single casting with the  less than 100 greater than  crystal orientation at the bonding interface aligned.
In addition, the present invention provides a method for manufacturing an airfoil portion of a turbine blade by joining a plurality of members formed of the same or different nickel base superalloys, wherein a pressure is exerted on the members to be joined so as to apply a pressure of 10 to 100 MPa to the bonding interface, a pulse voltage is applied to the members to pass an electric current therethrough so as to heat the joint portion to a temperature of 1000 to 1280xc2x0 C. by resistance heating, and the passing of electric current is continued to maintain the joint portion at that temperature for 5 to 60 min.
The turbine blade according to the present invention has a structure in which a plurality of members constituting the turbine blade are mated to each other, and the bonding interface has undergone diffusion bonding.
According to the present invention, a turbine blade formed of a nickel base heat-resistant alloy or alloys can be manufactured by joining a plurality of members without causing cracking at the joint portion or a lowering in material properties due to thermal effects, and reliability of the turbine blade is enhanced.